Recent Trends in Conjugated Polymer‐Based Thermoelectrics From Materials to Device

Recent Advances in Molecular Design of Organic Thermoelectric Materials

A Wavy-Structured Highly Stretchable Thermoelectric Generator with Stable Energy Output and Self-Rescuing Capability

As two pivotal functional segments, triazine and porphyrin can be coupled to form a highly cross-linked conjugated polymer. Although the obtained conjugated polymers are almost insoluble in most so...

The opportunities for creating the most effective thermoelectric (TE) material can be maximized by considering the full system for a given application when developing the material. If conversion efficiency is the only consideration in the design of a TE material, then maximizing average ZT over the largest temperature range may be the best choice. If the system or end application is unknown or not well defined, this design path may also be the best choice. However, there are more factors that should affect TE material design choice than just maximizing average ZT when considering the system-level attributes of an application. The following paper demonstrates how other aspects of the design affect how TE material properties (Seebeck coefficient, electrical resistivity, and thermal conductivity) can be tuned to get maximum performance for a given application. These other aspects include device and system level parasitic losses, constraints, and design objectives. Simulation results are provided that demonstrate how a TE material with a lower average ZT can be more effective depending on the design objective than a TE material with a higher average ZT.

Bismuth selenide (Bi2Se3) and bismuth telluride (Bi2Te3) are well-known compounds for thermoelectric (TE) cooling and generation applications near room-temperature. The performance of TE materials is quantified by a dimensionless figure of merit, ZT = α2σT/κ, in which α, σ, κ, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and absolute temperature, respectively. Currently, enhancing the TE power factor (PF = α2σ) of Bi2Se3 and Bi2Te3 thin-films remains a challenge due to the coupling amongst TE material properties and the difficulty of growing stoichiometric films under elevated substrate temperatures (Ts), at which is beneficial for enhancing the σ. In this thesis study, n-type TE Bi2Se3 and Bi2Te3 thin films were grown on SiO2/Si substrates using pulsed laser deposition (PLD). The effects of the structure, composition, and morphology on the TE properties of Bi2Se3 and Bi2Te3 thin films were investigated by controlling background ambient pressures (P) and Ts in PLD depositions. We found that the deposition in relatively high P (≥ 40 Pa) could obtain stoichiometric films at extended Ts up to 300 °C for Bi2Se3 and 340 °C for Bi2Te3, which can reduce the carrier concentration (n) and significantly enhance the Seebeck coefficient (α), following the α~n-2/3 relation approximately. Furthermore, at high Ts- growths, the obtained structures of highly (00l)-oriented – layered of large crystallites led to the substantial increase in the carrier mobility µ and thus improve the σ (= nµe). For example, the stoichiometric Bi2Se3 films grown at grown at 300 °C and 40 Pa with highly (00l) oriented and layered-hexagonal platelets possessed the highest PF of 5.54 µWcm-1K-2, where ׀α׀ = 75.8 µV/K and σ = 963.8 S/cm. Similarly, the stoichiometric Bi2Te3 films grown at Ts = 220–340 °C and PAr = 80 Pa with highly (00l)-oriented and layered structures showed the best properties, with a carrier mobility µ of 83.9 – 122.3 cm2/Vs, an ׀α׀ of 172.8 – 189.7 µV/K, and a remarkably high PF of 18.2 – 24.3 µWcm-1K-2. In contrast, the Te-rich films deposited at Ts ≤ 120 °C with (015)-preferred orientations and columnar–small grain structures or the Te-deficient film deposited at 380 °C with Bi4Te5 polyhedron structure possessed poor properties, with µ < 10.0 cm2/Vs, ׀α׀ < 54 µV/K, and PFs ≤ 0.44 µWcm-1K-2. This study provides a comprehensive understanding the interrelationships between PLD processing conditions, microstructures, and TE properties of Bi2Te3-based thin films, promising for further improving the TE performance of materials and applications. In brief, the morphology of highly (00l) oriented–layered large crystallite structures and the stoichiometry predominantly contribute to the substantial enhancement of µ and ׀α׀, respectively, resulting in remarkable enhancement in PF.

Thermoelectric (TE) material, as one of new energy materials, is regarded as one of the most important energy-saving materials, which can directly achieve the interconversion between heat and electricity. Presently, inorganic semi-conductors are considered to be the best thermoelectric materials. However, the development of new thermoelectric material has attracted great attention owing to the scarce resource and limited performance of inorganic thermoelectric materials. As the discovery and wide application of conducting polymers (CPs), organic thermoelectric materials have come into the sights of people over the past 30 years. The development of CPs as a promising thermoelectric material has gained great attention over the past decades. Various CPs have been developed and investigated on thermoelectric performance such as polyacetylene (PA), polyaniline (PANi), polypyrrole (PPy), polythiophene (PTh) and their derivatives since 1989. Among numerous CPs, poly(3,4-ethylenedioxythiophene) (PEDOT) shows many superior properties compared to others due to its excellent environmental stability, water solubility, easy processibility, and high electrical conductivity, which brings new strategy for studies of high performance organic thermoelectric materials. The conductive PEDOT combined with an insulating poly(styrenesulfonate) (PSS) can form a stable aqueous PEDOT:PSS with a good film-forming property and has been regarded as one of the most potential thermoelectric material. In 2008, the thermoelectric performance of PEDOT:PSS pellets were reported systematically for the first time. Although its thermoelectric figure-of-merit ( ZT ) was as low as 10 - 3, it was one of the highest values for CPs at that time. Soon afterwards, the thermoelectric performance of the free-standing PEDOT:PSS film achieved a ZT value as high as 10 - 2 in 2010. During these years, great attention focused on the derivatives of PTh, polyselenophene (PSh), PANi, and polycarbazole (PCz). Since 2010, PEDOT:PSS has come into sight of researchers in the world. The past few years witnessed great development of thermoelectric performance of conducting PEDOT:PSS ( ZT ~10 - 1). In recent ten years, the thermoelectric figure-of-merit ( ZT ) of PEDOT has been enhanced by three orders of magnitude from 10 - 4 to 10 - 1 as one of the most promising organic thermoelectric materials. Presently, the enhanced thermoelectric performance for PEDOT concern in the increased electrical conductivity via an easy method, especially for PEDOT:PSS. A large electrical conductivity of PEDOT:PSS thin film more than 3000 S cm - 1 has been achieved by adding an organic solvent or post-treatment with organic solvents, acid, and ionic liquids. Compared to inorganic thermoelectric materials, PEDOT:PSS can achieve a high electrical conductivity without an obviously decreasing Seebeck coefficient. A further improvement of thermoelectric performance for PEDOT:PSS has been devoted to the optimization of Seebeck coefficient via the pH value adjustment, the reduction of the oxidized level of conductive PEDOT, or composite with inorganic thermoelectric materials. A large thermoelectric power factor has become a reality. Although there are large gaps from actual industrial application for thermoelectric PEDOT ( ZT ˃1), yet most efforts focus on the high performance PEDOT. A large number of new techniques and methods have been developed to improve the thermoelectric performance of PEDOT. This review pays the attention to the advantages and characteristics, development history, and performance improvement of PEDOT as a promising organic thermoelectric CP from discovery to development. In order to achieve a high performance thermoelectric PEDOT, more attention should be paid to the development of low dimensional PEDOT crystal, control of oxidized level, extension of conjugated chain length of PEDOT, new preparation method and techniques, and the effects of crystal structure on electron transport properties as well as the flexible PEDOT thermoelectric devices in the future. Additionally, it is necessary to keep up with the development of n-type organic thermoelectric materials.

Physical and chemical properties of low-temperature nanostructured thermoelectric materials on the basis of Bi2Te2.8Se0.2 and Bi0.5Sb1.5Te3

Thermoelectric (TE) materials can directly realize the mutual conversion between heat and electricity, and it is an environmentally friendly functional material. At present, the thermoelectric conversion efficiencies of thermoelectric materials are low, which seriously restricts the large-scale application of thermoelectric devices. Therefore, finding new materials with better thermoelectric properties or improving the thermoelectric properties of traditional thermoelectric materials has become the subject of thermoelectric research. Thin film materials, compared with bulk materials, possess both the two-dimensional macroscopic properties and one-dimensional nanostructure characteristics, which makes it much easier to study the relationships between physical mechanisms and properties. Besides, thin film are also suitable for the preparation of wearable electronic devices. This article summarizes five different preparation methods of Cu&lt;sub&gt;2&lt;/sub&gt;Se thin films, i.e. electrochemical deposition, thermal evaporation, spin coating, sputtering, and pulsed laser deposition. In addition, combing with typical examples, the characterization methods of the film are summarized, and the influence mechanism of each parameter on the thermoelectric performance from electrical conductivity, Seebeck coefficient and thermal conductivity is discussed. Finally, the hot application direction of Cu&lt;sub&gt;2&lt;/sub&gt;Se thin film thermoelectrics is also introduced.

Polymer based thermoelectric nanocomposite materials and devices: Fabrication and characteristics

Thermoelectric (TE) materials with efficiencies comparable to Carnot efficiency are highly desirable for applications in devices that utilize the TE effect to convert heat into electricity. The efficiency of a TE material is measured by its figure of merit, which is influenced by the complex interplay between electronic and thermal transport parameters. Janus materials, with their large specific surface area, can enhance the movement of electrons and holes generated by heat towards the reaction interface. One example of a two-dimensional (2D) transition metal dichalcogenide used in this study is molybdenum-based materials. In this research, we investigated the electronic and thermoelectric properties of 2D Janus materials using first-principles electronic structure methods combined with semiclassical Boltzmann transport theory, calculating relaxation times using molybdenum-based materials and analyzing their impact on TE properties. We found that relaxation time calculations played a critical role in significantly altering transport phenomena, leading to a higher figure of merit and thus enhancing TE efficiency. This study is significant as it highlights the important structure-property relationship in determining TE efficiency, which could pave the way for the discovery of new TE materials with higher efficiencies. The results show that the thermoelectric properties of molybdenum-based materials indicate that MoSeO exhibits the highest maximum power factor (PF) across all temperatures. For p-type carriers in MoSeO, the PF reaches 32.568 mW/mK2 at 900 K, while for n-type carriers, the maximum PF reaches 3.419 mW/mK2 at the same temperature. These findings suggest that MoSeO has a high thermoelectric potential, particularly at high temperatures, making it a promising candidate for future thermoelectric applications.

We relate the static charging of organic field effect transistor (OFET) gate dielectric polymers, the resulting change in threshold voltage shift, and the modulation of thermoelectric properties of the transistor semiconductor. We utilize a bottom gate OFET structure with cross-linkable polystyrene and plain polystyrene (XLPS/PS) bilayer dielectrics and dinaphthothienothiophene (DNTT) organic molecular semiconductor with tetrafluorotetracyanoquinodimethane (F4TCNQ) doping layer. The conductivity and Seebeck coefficient are measured before and after charging. We find that the conductivity increases significantly, while the Seebeck coefficient decreases correspondingly in a trend of S ∝ ln(σ) after the dielectrics were negatively charged, resulting in an average of 5-fold increase in power factor at the maximum charging level. Additionally, temperature-dependent conductivity measurements show that the activation energy decreases with the increasing conductivity. We calculate the Seebeck coefficient based on the activation energy and find the result is consistent with the Mott mobility edge model. This work confirms the charging mechanism in our previous published work, including the location of static charges in the bulk of the dielectric, and demonstrates modulation and enhancement of the thermoelectric properties of organic thermoelectric materials by adjacent chargeable dielectrics without changing the molecular microstructure or applying external gate voltage during operation.

Computational advances for energy conversion: Unleashing the potential of thermoelectric materials

Since the discovery of the Seebeck effect in the early 19th century, which describes the electromotive force depending on the temperature gradient, thermoelectric (TE) materials have been studied in the research of the alternative energy source. Especially, high-efficiency TE materials are important for power generation device within 200 ~ 500 K temperature range because it has simple and reliable structure [1]. Inducing the nanotechnology to the synthesis of TE materials in the 1990s, TE materials have achieved lower thermal conductivity without reducing the electrical conductivity and it has improved thermoelectric figure of merit (ZT) dramatically [2]. It has been researched reduction of dimension like nanowire (1D) and superlattice (2D), or nanocomposition with nanoparticle embedded in particular to improve the ZT by inducing the phonon scatter at the interface between particle and grain boundary. Despite of the good performance of low dimension TE materials, commercial measuring equipment is not appropriate to measure the thermal properties of these materials. Consequently, various methods were suggested to evaluate the noble TE materials depending on the dimension of the materials [4 ~ 7]. Unlike the bulk materials, TE materials which have low dimension such as 2D, 1D and 0D, or which nanocomposition technique was induced, has different thermal properties of in- and cross-plane directions because of the anisotropic crystal structure and array of quantum dot [8]. So many research groups have developed various types of evaluation methods which were appropriate for their research results. These methods could measure the thermal properties in the only one direction between in- and cross-plane. And these methods were insufficient to have a standard traceability because those didn’t apply to other cases. In this study, we developed a sensor unit which could measure the Seebeck coefficient in the in- and cross-plane directions. This sensor unit was designed to measure the TE sample generated by various deposition methods. Target sample was micro scale film type which was general results of nanocomposition and superlattice TE sample. In case of physical vapor method (PVD) such as sputtering and evaporation, we designed that TE material could be deposited on the sensor to evaluate the properties more exactly. While the commercial measuring equipment was difficult to measure the thermal properties in case of film type TE materials because the temperature gradient become smaller as film thickness thinner, we measured Seebeck coefficient of 1 ~ 100 ㎛ thickness TE sample. And we measured the coefficient at 300 ~ 500 K temperature range which TE material has merit compared to mechanical heat engines to retain the availability of the sensor. To confirm the validity of the sensor, we measured Bi-Te and silicon sample which have progressed preceding research from other research groups. Reference [1] Tritt, T.M., (2011) Annual Review of Materials Research, 41, pp. 433-448. [2] Dresselhaus, M.S., Chen, G., Tang, M.Y., Yang, R., Lee, H., Wang, D., Ren, Z., Fleurial, J.-P., Gogna, P., (2007) Advanced Materials, 19 (8), pp. 1043-1053. [3] Hicks, L.D., Harman, T.C., Dresselhaus, M.S., (1993) Applied Physics Letters, 63 (23), pp. 3230-3232. [4] Harman, T.C., Taylor, P.J., Walsh, M.P., LaForge, B.E., (2002) Science, 297 (5590), pp. 2229-2232. [5] Zeng, G., Zide, J.M.O., Kim, W., Bowers, J.E., Gossard, A.C., Bian, Z., Zhang, Y., Shakouri, A., Singer, S.L., Majumdar, A., (2007) Journal of Applied Physics, 101 (3), art. no. 034502, [6] Ohta, H., Kim, S., Mune, Y., Mizoguchi, T., Nomura, K., Ohta, S., Nomura, T., Nakanishi, Y., Ikuhara, Y., Hirano, M., Hosono, H., Koumoto, K., (2007) Nature Materials, 6 (2), pp. 129-134. [7] Shin, H.S., Jeon, S.G., Yu, J., Kim, Y.-S., Park, H.M., Song, J.Y., (2014) Nanoscale, 6 (11), pp. 6158-6165. [8] Vineis, C.J., Shakouri, A., Majumdar, A., Kanatzidis, M.G., (2010) Advanced Materials, 22 (36), pp. 3970-3980. [9] Vining, C.B., (2009) Nature Materials, 8 (2), pp. 83-85.

Effective approaches to produce high performance single-walled carbon nanotubes/platinum based hybrid films by inserting thermoelectric material with high seebeck coefficient

Influence of oriented CNT forest on thermoelectric properties of polymer-based materials